My system is a Trane split XV95-100btu/XL15i-4ton.
For historical reading you may go to the following link.
But here is a summary.
My system started with a total external static pressure of ~1.1 (on cool) with 3 trunks coming off the supply plenum. A 12x9, a 12x8, and a 8x9.5 all leading to a total of 16 supply runs where three were 6" round and 13 were 5" round...all metal ducts (no flex).
I am happy to say I am now at 0.75 tesp (down from 1.1) and the upstairs is within a degree or 2 of the downstairs (was 12 degrees hotter).....without tearing out any drywall and leaving my small 5 or 6 inch wall stacks in place.
I realize now after the fact that the problem was my system was likely oversized.
Here is the list of things that they did to ultimately resolve the problem.
I'm sure some contributed more than others.
1. Converted the 12x8 trunk off the plenum to a 14x14.
2. Converted the 8x9.5 trunk off the plenum to a 16x8.
3. Changed the return drop from a 24x10 to a 24x20.
4. Added a 5" media filter 25x20 horizontial in the return drop to replace the 1" restrictive electrostatic OEM filter located right at the blower.
5. opened all the registers and removed the "restrictor" plates that were designed to increase "throw" by adding static pressure.
6. Increased 2 upstairs return grills from 10x6 to 14x14.
7. Sealed in between the drywall and the register/return grills with mastic or foil tape.
8. Sealed the return and supply leaks in the basement around the furnace/coil as well as in the ductwork.
9. Added R-30 insulation batts to the attic.
10. Cut the Bk jumper on the furnace board to reduce airflow to 80%. This effectively makes the system a 3.2 ton instead of a 4-ton....delivering 1280 CFMs instead of 1600. The ironic part is I never got 1600 CFM anyway because the static was so high I got closer to 1250-1300 CFM.
~1300 CFM cools my house very well.
According to the response I got from Trane, the system can be run up to static pressures of 0.9 (with a reduction in CFM due to the static).
For heating, the dip switches were just placed on the lowest fan-speed where stage 1 -is 900CFM and stage 2 is ~1300CFM.
Thanks to all on this site who contributed to the resolution.
I'd like to add some special thanks to ......well you all know who you are.